Movable mechanical structures

ABSTRACT

Several improvements relating to hexapods are described. The improvements relate to universal joints and ball and socket joints where two struts meet at a common focal point, two axis gimbals which are used for ballscrew drives in universal joints and a micro positioning device incorporating a hexapod structure. The universal joint (1) includes two part spheres (13, 14) connected to one strut (12) and a rotatable ring (15) connected to another strut (11). The two part spheres (13, 14) are secured together by fasteners (25) with the ring (15) secured therebetween so as to form a sphere. The ring (15) is preloaded and rotatable about a common axis (17) of the sphere.

FIELD OF THE INVENTION

This invention concerns improvements relating to movable mechanicalstructures particularly though not exclusively to parallel strutstructures such as hexapods or Stuart platforms. More specifically, theinvention concerns universal joints and ball and socket joints where twostruts meet at a common focal point, two axis gimbals which are used forballscrew drives in universal joints and a micro positioning deviceincorporating a hexapod structure.

BACKGROUND OF THE INVENTION

In known machine tool design, it has been necessary to devise jointsincorporating a mechanism able to constrain the relative rotationbetween two struts whilst permitting them to pivot on a common axis,thus allowing the couplet to be free to articulate in all axes. Thesetypes of joints are necessary for building parallel strut structures,such as hexapods.

Methods have been proposed where the universal freedom about a pivotpoint is enabled in the form of a ball and socket joint; where eachstrut is connected to a hemisphere together defining the ball of saidjoint. However, these previous solutions to this problem have proveddifficult to build accurately because of the requirement to assemble anaccurate sphere out of two hemispheres. It is much easier to build asingle surfaced sphere which can be ground and/or lapped to achieve thedegree of sphericity required.

Modern machine design such as hexapod based machine tools may requireuniversal joints capable of a very wide degree of freedom.

A ball and socket joint is conceptually a good solution because it canbe made to high precision and is easy to calibrate because it accuratelymaintains its pivot origin. The disadvantage of a conventional solutionis the limited degree of freedom that can be supported because of thenecessity of holding the ball over a large angle to minimise holdingpressures and hence friction.

One solution that has been proposed is to retain the ball magneticallyin the bottom half of the socket only, thereby leaving more than ahemisphere of articulation space. While being suitable for someapplications, the joint would become unduly massive to achieve the veryhigh holding forces necessary in some cases such as for large scalemachining.

In the design of hexapods, it has been necessary to provide a mechanismwell suited to drive a screw shaft through a focal point notionallywithin a two axis universal joint.

To build a ballscrew drive point in a universal joint it is necessary toconstrain its motion to two degrees of freedom so that it can resist thetorque of rotating the ballnut. This can currently be achieved either bya standard two axis gimbal with orthogonally opposed axes or by asuitably constrained ball and socket joint. Both of these methods havedisadvantages.

In the case of the standard gimbal, because preload needs to be appliedto each end stop, any thermal or load stresses will move the pivot pointabout both orthogonal axes. This makes the node point unreliable.

In the case of the constrained ball and socket joint, when confrontinglarge loads, stiffness can only be achieved at the cost of friction inthe ball to socket interface; and/or extending the socket over more ofthe ball thereby limiting the articulation.

There is a need for a mechanism able to translate a load through sixdegrees of freedom at extreme resolution. On many occasions it isdesirable to alter the position and alignment of instrument componentsby very small degrees. Presently this is accomplished by multi-axisstages arranged in a series chain of polar and Cartesian single axisstages. It is difficult to keep such an arrangement stiff without undulyincreasing the size and weight of the structure and reducing its freedomof available movement.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the invention to overcome or substantially reduce atleast some of the above described problems associated with movablemechanical structures.

According to a first aspect of the present invention there is provided auniversal joint for connecting together two struts, said universal jointcomprising two part spheres connectable to one strut and a rotatablering connectable to the other strut, said two part spheres being securedtogether with said ring secured therebetween to form a sphere and saidring being rotatable about a common axis of said sphere.

According to a second aspect of the present invention there is provideda combination of a universal joint and two struts, said universal jointcomprising two part spheres secured together and a rotatable ringsecured therebetween to form a sphere, one of said struts beingconnected to said part spheres and the other being connected to saidring, such that both of said struts can pivot relative to each otherabout a common axis of said sphere.

According to a third aspect of the present invention there is provided aball and socket joint for pivotally connecting together two struts at acommon focal point, said ball and socket comprising a ball held in a twopart socket, said two part socket comprising a reference portion havinga hemispherical inner surface for engaging the ball and a retainingportion for retaining the ball in the socket, the retaining portionbeing slidable on said reference portion and having means for engagingthe ball.

According to a fourth aspect of the present invention there is provideda two axis gimbal having a single axis preload force applied tocomplementary annular surface formations provided about a single axisthereof.

According to a fifth aspect of the present invention there is provided atwo axis gimbal comprising primary means for rotating a strut connectedthereto in a first axis and secondary means for rotating the primarymeans coupled thereto in a second axis orthogonal to said first axis,wherein the secondary means includes preloading means for preloadingsaid gimbal in said second axis and said preloading means includes africtional interface of complementary surface formations.

According to a sixth aspect of the present invention there is provided amicro positioning device comprising a base, a platform, and a pluralityof elongate struts each connected at its respective ends to said vaseand said platform, said plurality of struts being arranged to providesix degrees of freedom of movement between said base and said platform,wherein said each of said struts comprise Magnetostrictive,electrostrictive or piezoelectric material and are linearly extensibleby the application of an electromagnetic field thereto.

The above and further features of the invention are set forth withparticularity in the appended claims and together with the advantagesthereof, will become clear to those skilled in the art fromconsideration of the following detailed description of several exemplaryembodiments of the invention given with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a universal jointembodying a first and second aspects of the present invention;

FIG. 2 is a schematic perspective view of the respective parts of theuniversal joint shown in FIG. 1;

FIG. 3 is a schematic sectional view of a ball and socket jointembodying a third aspect of the present invention at a maximal angularposition;

FIG. 4 is a schematic sectional view of a ball and socket joint of FIG.3 at a minimal angular position;

FIG. 5 is a schematic perspective view of a two axis gimbal embodyingfourth and fifth aspects of the present invention; and

FIG. 6 is a schematic perspective view of a micro positioning deviceembodying a sixth aspect of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Each of the following embodiments is described separately hereinafterbut it is to be appreciated that they are all related to movablemechanical structures by being parts of a hexapod structure.

UNIVERSAL JOINT

Referring now to FIGS. 1 and 2 there is shown a universal joint 1including a ball joint sphere 10 connected to first and second struts11, 12. The ball joint sphere 10 is assembled out of two parts 13, 14and prepared to a high accuracy. One of the parts 14 is connected to thesecond strut 12 whilst the first strut 11 is connected to a ring 15which is retained in a circumferential channel 16 formed about arotational axis 17 of the sphere 10.

The universal joint 1 is assembled by the sphere 10 being opened and thering 15 being introduced into the channel 16. The sphere is thenreassembled to recreate the sphericity with the second strut 11 able torotate about the sphere's axis 17.

The sphere 10 and strut 12 are manufactured by turning an appropriateworkpiece in the axis of the second strut 12. The resultant sphere 10and strut 12 are then turned again, but in an orthogonal axis 17 tocreate the shell shown in FIG. 2 (left). The second strut 12 is cut awayat 18 like a pair of scissors to merit a tight closure angle betweenboth struts 11, 12 when the universal joint is assembled. A bearingsurface 19 of one of the parts 14 is an annular groove designed to matewith a conforming counter surface 20 of the ring 15. Alternatively itcould be raceway to retain balls or rollers. The key design objective isthat it can support axial loads effected by the preload when assembledand radial loads caused by compression or tension forces in the strut.

The first strut 11 is similarly cut away at 21 to enable closure. Thering 15 features the other bearing surface 20, such that when preloadedtogether with the sphere 10, it is able to rotate about the sphere'saxis 17. When the sphere 10 is closed, a further annular pressure plate22 is configured to act against the ring 15, pressing it together withits mating bearing surface 19 by springs 23 or other loading devicelocated in pockets 24. The act of closing the sphere 10 introduces thispreload.

The two parts 13, 14 of the sphere 10 are located by kinematic meanssuch as a Kelvin Clamp such as the type disclosed in FIGS. 27(a) and27(b) of U.S. Pat. No. 5,575,597, to ensure repeatable positioning afteropening to introduce the strut/ring. A set (in this embodiment 6) offasteners 25 are used to bolt the two parts together.

The inventive concept underlying this embodiment involves the majorityof the sphere's surface being connected to one of the two struts. Theparts 13, 14 of the sphere are hollowed out and this advantageouslyenables the mass of the universal joint 1 to be minimised.

BALL AND SOCKET JOINT

Referring now to FIGS. 3 and 4, there is shown a ball and socket jointembodying the third aspect of the present invention. The ball and socketjoint 30 comprises a ball 31 a reference socket 32 and a movable partialsocket 33. The ball 31 of this embodiment preferably comprises theuniversal joint 1 described hereinbefore with reference to FIGS. 1 and2. The reference socket 32 comprises hemispherical shell is preparedwith a precision internal surface 34, and a reasonably good annularexternal surface 35, extending down to a mounting flange 36.

The partial socket 33 is prepared such that it has a low frictioncontact ring 37 conforming to the sphericity of the external surface 35of the reference socket 32. The partial socket 33 is modified into acylindrical profile at it supper end 38, such that is necessary to allowit to pass over the lip of the reference socket 32 during installation.

An annular pressure ring 39 is prepared such that it has an internalprofile 40 confirming to the retained ball 31, and a flange 41 extendingto overlap with the partial socket 33.

The ball and socket joint 30 is assembled by retaining the pressure ring39 against the partial socket 33 with a radial array of fasteners 42,each acting through a stack of disc springs 43. The compressionestablished in these springs 43 during assembly creates the ballretaining preload. The pressure ring 39 is allowed to effectively'float' on its springs 43 so as not to fight with the tracking accuracyestablished by the reference socket 32.

The internal space or cavity 44 between the socket parts and ballmaintains a constant volume during joint articulation and isbeneficially filled with a lubricant. This lubricant could be a shearviscous lubricant such as Kilopoise. Lubricant leakage could beminimised with sliding annular seals acting between ball and pressurering and/or partial socket and external reference socket. A lubricantpressure feed could be established into this cavity to maintain constantlubricant film thickness.

The reference socket and pressure ring contact surfaces 34, 40 can bemade by replicating the profile of a precision gauge ball with a resincomposite such as Moglise. In this way a very high precision device canbe made at low cost.

The external reference socket surface 35 and contacting ring 37 of thepartial socket 33 need not be prepared to such high precision.Inaccuracies can be compensated for by the array of disc springs 43causing the holding preload against the pressure ring 39, whichreferences itself against the ball.

In this embodiment, the ball is retained in a precise positionalreference socket by an annular pressure ring. This is preloaded to trapthe ball by tensioning it against a further partial socket shell trappedby the outer surface of the reference socket shell.

In use, to achieve maximum angular movement, the strut(s) 45 first comesup against the lip of the socket, and the displaces this further as thepartial socket slides around the reference socket. At all times thepressure ring maintains its holding force pushing the ball into thereference socket.

The effective total socket angle can be less than 180 degrees being thetotal of the included angle of the reference socket plus the two contactangles of the pressure ring. A further advantage of this embodiment isthat extended articulation of the struts 45 is permitted without the useof magnetic forces.

TWO AXIS GIMBAL

Referring now to FIG. 5, there is shown a two axis gimbal embodying thefourth and fifth aspects of the present invention. The two axis gimbal50 comprises a barrel 51 having a drive strut 52 extending from a sidesurface 53 thereof. The barrel 51 is constrained within two clampingstructures 54 which act to hold the barrel 51 and to restrict itsmovement to rotation about a local axis 55, hereinafter referred to asthe secondary axis.

The barrel 51 has two circumferential grooves 56 extended around itsside surface 53. Complementary ridge formations 57 provided on innersurfaces of said clamping structures 54, engage with the grooves 56 toconstrain the barrel movement to rotation about the secondary axis 55.

Each of the clamping structures 54 is provided with a series ofconcentric continuous annular grooves 58 which are provided about afixed axis 59 hereinafter referred to as the primary axis. Respectiveclamping heads 60, also provided along the primary axis 59, havecomplementary concentric continuous annular ridges or tapers 61 whichare arranged to engage the grooves 58 of the clamping structures 54. Inthis way, rotation of the clamping structures 54 is possible withrespect to the primary axis 59.

The tapers 61 of the clamping heads 60 and the grooves 58 of theclamping structures 54 enable a single axis preload to be applied alongthe primary axis 59 which provides support for the gimbal against bothradial and axial forces.

The primary axis 59 needs to sustain large radial force as a result ofthe tension or compression in the drive strut 52 (shown in FIG. 5protruding vertically). Axial forces are largely as a result of thepreload applied across the barrel 51 or when the secondary axis 55 isrotated.

The secondary axis 55 needs to sustain large radial forces. If the drivestrut 52 acts in tension or compression only, then axial forces areminimal. The groove design can reflect this with shallower features. Infact, many alternative groove or taper designs are possible, theprinciple being to support both axial and radial forces.

The frictional interfaces (grooves 56, 58 and tapers 57, 61) can becoated with a bonded lubricant such as Molycote, a proprietary compoundcontaining Molybdenum. The large contact area promotes dampingespecially when the interface is additionally coated with a suitablesilicon lubricant such as a Fluorosilicon.

There are three areas of improvement which this embodiment has overconventional gimbals:

Both axes are supported from adjacent points, thereby avoiding bendingmoments in the usual gimbal rocker frame.

Friction pads are used in preference to bearings to enable higherpreloads and to damp out vibrations.

Thermal expansion acts along one axis only; thereby holding a morepredictable centre in all operating conditions.

There are three areas of improvement which this embodiment has over aconventional constrained ball and socket joint:

The proposed joint can hold the ball over a greater angle than a socketcan, without incurring any loss of articulation.

Movement is confined to slip-ways which are more readily sealed than acomplete socket.

It is easier to grind a precision groove than achieve high precisionsphericity in a large ball.

In an alternative embodiment, instead of using plain bearings, ball orroller bearings can be employed in the case of the primary axis, thesecan be of conventional radial design. For the secondary axis, because ofthe need to apply preload between both sides of the bearing, the bearingwould beneficially be split into two semicircular halves with anelastomeric coupling between said halves to enable the movement of ballsor rollers from one half to the other. Alternatively each half could bekept discrete with the balls or rollers forced to recirculate through aninternal return path (in a similar manner to a recirculating ball trackor nut).

MICRO POSITIONING DEVICE

Referring to FIG. 6 there is shown a 6 axis micro positioning device 70embodying a sixth aspect of the invention. The micro positioning devicecomprises a base 71 and a platform 72 mounted on the base 71 by 6supportive legs 73. The effective length and angular orientation of thelegs 73 is controllably adjustable to effect movement of the platform72.

The supportive legs 73 are arranged in 3 pairs and are coupled to theplatform at triangularly spaced apart locations 74, 75, 76. The legs 73extend from these locations 74, 75, 76 in divergent directions and areattached to the base 71 at spaced apart locations.

Each leg 73 comprises a magnetostrictive material, thoughelectrostrictive and piezoelectric materials are also suitable. Theextension of each leg 73 is caused by the application of a magneticfield along its length and electric coils 77 are provided around eachstrut for generating the respective magnetic fields.

In general terms, this embodiment integrates the desired movement into aparallel strut mechanism often referred to as a Stuart Platform orhexapod, using struts 73 comprised of magnetostrictive electrostrictiveor piezoelectric materials. Devices of this type are of use to laseroptics, integrated circuit wafer preparation, tunnelling microscopynano-mechanics etc.

Magnetostrictive materials have been developed which exhibit relativelylarge expansions; such as Turfenol available from Johnson Matthey, whichcan expand by about 0.15%. The extent of the expansion can be controlledvery smoothly by using the electrified coil 77 around each strut 73 togenerate a variable magnetic field.

By controlling the current to say 16 bits (1 in 65.5k) struts of 40 mmlength can be usefully stepped through length increments of 10nanometers (0.01 micron) over a useful range of 0.06 mm. Higherresolutions can be obtained by shortening the effective strut oremploying piezo based systems. Both options are however at the expenseof useful range.

These increments would gear up by virtue of the hexapods geometry, butwould alternatively be error averaged such that a small error in anindividual strut length will equate to an even smaller error in theplatform.

The struts could be connected between the base and platform by variousmeans depending on loading and translation requirements.

If only the smallest platform movements are required, the struts can bebonded between base and platform. The arrangement then relies on strutflex to accommodate the changes in geometry. Because of the resultantcomplex virtual pivot points, the structure needs to be calibrated toachieve good absolute accuracy. Other sorts of flexural or elastomericbase and platform joints can be applied to reduce strut flexing.

For larger translations, or where volumetric accuracy is at a premium,spherical universal joints can be employed. A magnetic socket can beused to retain the spherical end to the strut, resting unambiguously onthree kinematic pads. By explicitly measuring the spherical surfaces asthey seat in their kinematic sockets, the pivot points can beestablished. These geometry nodes are used to determine accurateplatform translations. When platform positions are compared against anabsolute reference, the good initial calibration makes it much easierfor subsequent error correction software to converge.

If heavy platform loads or high accelerations are envisaged, socketscomprising mounting pads to both seat the ball and retain it can beproduced. To be unambiguously constrained they would have three lowfriction kinematic pads arranged such that their normals intersect at acommon point consistent with the radius of the sphere to be retained.These pads could consist of polymeric bearing material or Zirconium orother low friction ceramic material. A preloaded thrust ring consistingof at least three more bearing surfaces is then retained around thestrut protruding from the sphere acting against the sphere in such a wayas to retain it in its socket.

As an alternative to holding the sphere between kinematic pads, a largerconforming spherical concavity can be retained, and repeatablepositioning of the enclosed sphere obtained by surface error averagingwith a suitable non Newtonian lubricant (or one otherwise sealed toprevent it being squeezed out). Other options exhibiting good averaginginclude air or hydrostatic bearings.

Methods to measure the strut length explicitly in order to close acontrol loop could be employed. In one embodiment LVDT's are used. Thesecould act through a hollow core in the struts with a suitablydimensionally stable push rod and mounting. Alternatively they could actlike a cylinder enclosing the magnetostrictive drive coils.

Other solutions are possible depending on resolution required includingoptical interferometric means.

The control system for such a device consists of processor able totranslate from Cartesian space to hexapod strut lengths. It employs acalibrated correction table which equates a given current to the coil toa strut extension. This also includes thermal correction based on themeasured temperature of the strut. It could alternatively be based onclose loop control if suitable absolute strut length or other positiontransducers are fitted. High level software enables Cartesian and polarincrementing, 6 axis path following and pattern scanning to be achieved.

It is noted that if the node geometry is such that adjacent strut pairsform parallelograms (with their base separation the same as the platformseparation) and the struts forming the parallelograms are controlled insynchronous to common lengths, then the platform will be constrained tothree (X, Y, Z, ) degrees of freedom only.

Thus having described all the aspects of the invention by reference tospecific embodiments, it is to be well appreciated that the invention isnot limited to the above described embodiments but that modificationsand variations are possible without departure from the spirit and scopeof the invention as determined by the appended claims.

I claim:
 1. A universal joint connecting together two struts, saiduniversal joint comprising two part spheres connected to one strut and arotatable ring connected to the other strut, said two part spheres beingsecured together with said ring secured therebetween to form a sphereand said ring being rotatable about a common axis of said sphere,preload means for pressing said ring against one of said part spheres toprovide preloading of said ring, wherein said preload means comprises anannular pressure plate and a plurality of loading devices, each of saidplurality of loading devices being located in a pocket of the other ofsaid part spheres.
 2. A universal joint according to claim 1 whereinsaid ring and one of said part spheres each comprise a complementarybearing surface permitting rotation therebetween.
 3. A universal jointaccording to claim 1 wherein said complementary bearing surfacescomprise annular groove and ridge formations.
 4. A universal jointaccording to claim 1 wherein the two part spheres are secured togetherby a plurality of fasteners extending therebetween.
 5. A universal jointaccording to claim 1 wherein each of the part spheres is hollow.
 6. Auniversal joint as claimed in claim 1, wherein the preload devices eachcomprise a spring.
 7. A universal joint according to claim 1 whereinsaid struts further comprise complementary cut portions which allow arelatively small closure angle to be attained between the struts.
 8. Auniversal joint according to claim 1 wherein a ball constituted by saidtwo part spheres and said ring is held in a two part socket, said twopart socket comprising a reference portion having a hemispherical innersurface for engaging the ball and a retaining portion for retaining theball in the socket, the retaining portion being slidable on saidreference portion and having means for engaging the ball.
 9. A ball andsocket joint according to claim 8 wherein an internal space between thereference portion, the retaining portion and the ball is filled withlubricant to provide a liquid bearing.
 10. A universal joint accordingto claim 9 wherein said lubricant comprises a shear viscous lubricant.11. A universal joint according to claim 8 wherein said referenceportion includes a mounting flange having mounting formations therein.12. A universal joint according to claim 8 wherein said referenceportion has a part spherical outer surface and said retaining portionhas a complementary part spherical inner surface for engaging the outersurface of the reference portion.
 13. A universal joint according toclaim 8 wherein said engaging means comprises an annular flange having aball engaging surface conforming to the curvature of the internalsurface of the reference portion.
 14. A universal joint according toclaim 13 wherein said annular flange is mounted to said retainingportion by a plurality of fastener means so as to generate a ballretaining preload.
 15. A universal joint according to claim 13 whereinthe ball engaging surfaces of said reference portion and said annularflange comprise a resin composite material.